Conventional roller bearing three-piece trucks have been standard railway industry equipment, especially in freight service, for many years. Such recent developments as higher center of gravity cars, heavier load limits and higher car operating speeds have resulted in significantly increased average and peak axle loads which are of particular significance as regards truck tracking performance. For example, in instances where a conventional three-piece truck is traveling on curved track, the conical wheels may not track around the curve but rather may have a tendency to slide on the rail head with severe flange rubbing. Such sliding, especially with large axle loads, can result in undue wear of the truck wheels and track components, and lateral track misalignment among other adverse consequences.
The art describes a variety of truck arrangements, often referred to as steering trucks, which are intended to permit the truck wheelsets to track without experiencing sliding or undue flange rubbing in curves. For example, Report No. 5576-78, issued in November, 1978 by the Department of Research--Canadian Pacific Limited, describes several such trucks.
Examples of other art which may be germane to steering truck arrangements are U.S. Pat. Nos. 4,202,276; 4,173,933; 4,170,179; 4,166,611; 4,151,801; 4,136,620; 2,134,343; 4,064,069; 3,948,188; 3,528,374; 3,517,620; 3,254,610; 2,956,515; 2,908,233; 2,756,688; 2,722,791; and 2,207,848.
Some prior steering truck arrangements attempt to isolate the side frames from the wheelset axle bearings in a manner to permit limited wheelset freedom and provide controlled restraint of the resulting wheelset excursions to thereby permit the wheelsets to yaw independently and track around curves without sliding and flange rubbing. However, permitting such independent wheelset yaw freedom for truck steering can also increase the tendency toward truck hunting.
Hunting in railway vehicles is the unstable cyclic yawing of trucks, and the resultant lateral oscillation of the railway car when the truck and car body oscillations become dynamically coupled. The hunting phenomenon is of particular significance in empty cars traveling at relatively high speeds, for example in excess of 40 miles per hour. Lateral track irregularities and the generally sinusoidal truck movement produced by conventional coned wheel geometry results in lateral wheelset excursions as each side of the wheelset alternately moves ahead of the other. This in turn can cause the wheel flanges to impact and rub against first one rail and then the other. These lateral impact loads can cause undesirable lateral car body oscillations and excessive truck component and rail wear. As the wheel treads and flanges wear, the wheel tread conicity becomes more severe and the flange-to-rail clearance increases thereby permitting even larger lateral excursions of the wheelsets during hunting and more severe dynamic response at lower speeds. In the extreme, lateral excursions can become severe enough to precipitate truck derailment.
Various improvements intended to control or eliminate such hunting responses have been developed. See for example my U.S. Pat. Nos. 4,080,016, 4,915,031 and Re. 31,784.
Such art as mentioned immediately above discloses devices which act at the side bearing locations to inhibit car body motions by directly restraining relative motion between the car body and the truck.
The prior art relating to steering trucks does in some instances address the matter of hunting problems which are aggravated by freeing the wheelset axle for movement with respect to the truck side frames. For example, U.S. Pat. No. 4,512,261 discloses an arrangement which reduces the lateral wheelset-to-side frame restraint, thus in turn reducing the wheelset-to-car body restraint. The side frame-to-wheelset freedom in this last mentioned patent affords sufficient longitudinal freedom for steering and reduced car body coupling by virtue of the lateral wheelset freedom.
The problem of hunting due to wheelset yaw has also been addressed in the prior art by truck arrangements providing articulating structures such as linkage arrangements which connect the wheelsets to force the linked wheelsets to move with respect to each other in predetermined ways. See for example U.S. Pat. Nos. 4,136,620, 4,067,262, 4,067,261 and 4,151,801.
The art has not been entirely successful in providing both steering and hunting control for railway trucks owing to the variety and complexity of the causes of these phenomena, including excessive component wear and/or maintenance problems, and exposure of link elements (in the case of wheelset articulation devices) to adverse operating conditions.